The invention relates generally to echo cancellers, and it relates, in particular to a double talk detector and a line noise detector for improved echo cancellation.
Echoes present a problem for long distance, two-way communication systems. In such a system, a station both receives a signal for its own use and transmits another signal containing different information. However, the received and transmitted signals, although they are supposed to be independent, are often not completely isolated. The result is that the received signal is re-transmitted at a lower level along with the intended transmitted signal. Because of the long transmission path between two stations, one station hears a delayed echo of its own transmission in its received signal. This echo has passed through the station with which it is in communication or an intermediate station.
Echoes are particularly noticeable when only one station is talking because the attenuated echo is not masked by a larger transmitted signal. A conventional method of eliminating echoes is an echo suppressor which disconnects the transmitting side of a station when no high level signal is present on the transmit side. One such echo suppressor system is described by Skrovanek et al in U.S. Pat. No. 4,115,668. Echo suppressors have found wide use in the telephone industry but suffer from several drawbacks. It is not uncommon for both talkers in a telephone conversation to be simultaneously talking. Accordingly, a double talk detector is often used to detect high level signals on both the transmit and receive lines. When the double talk condition is detected, echo suppression is removed by opening up the transmit line. However, the echo path is then re-enabled and an echo can often be heard within the transmitted signal.
In order to avoid the problems with echo suppressors, echo cancellers have been developed in recent years which, instead of breaking the transmit line, subtract the echo from the transmitted signal. An example of a telephone system using echo cancellers is shown in block diagram form in FIG. 1, in which their are two nearly identical stations, a west station 10 and an east station 12. Long distance communication between the two stations 10 and 12 is performed on unidirectional transmission paths 14 and 16. These channels 14 and 16 may be terrestrial wires or microwave channels or may involve communication satellites. A telephone at the west station 10 is connected into the system by a two-wire circuit 18 which is connected into four-wire circuit lines 20 and 22 through a hybrid 24. The four-wire lines are interfaced to the transmission channels 14 and 16 through a codec which performs a number of functions in the telephone industry, such as channel selection and momentary disconnection of four-circuit lines which are momentarily silent in order to increase the capacity of the transmission channels. Also connected to the hybrid 24 is a balancing network 28 which is intended to balance the impedance of the two-wire circuit 18. If the hybrid 24 is working perfectly, the received signal on the line 22 is completely diverted to the two-wire circuit 18 and is thus completely isolated from the outgoing line 20 for the transmitted signal. In this optimum case, there is no echo arising through the hybrid 24. However, because the impedance on the two-wire circuit 18 tends to vary and for other reasons, the hybrid 24 does not completely isolate the receive line 22 and the transmit line 20. As a result, an attenuated echo passes through the hybrid 24.
In order to prevent the re-transmission of such an echo, an echo canceller is inserted on the fourwire circuit. The signal x on the receive line 22 is connected to the input of an adaptive finite impulse response filter (AFIRF) 30. The AFIRF is well known and is described in an article by the inventor entitled "Echo Canceller With Adaptive Transversal Filter Utilizing Pseudo-Logarithmic Coding" appearing in Comsat Technical Review, Vol. 7, No. 2, Fall 1977 at pages 393-428. The AFIRF 30 adaptively models the response of the hybrid 24 and its associated connections to create an echo of the received signal so that when the received signal x is fed to its input, the filter produces a signal y' on its output which is a prediction of the echo. The predicted echo y' is connected to an inverting input of a summer 32. A noninverting input of the summer 32 is connected to the transmit line 20 which carries a signal y which is the sum of the echo and the intended transmit signal. The output of the summer 32, the difference y-- y' of the transmit signal and the predicted echo, is fed back to the AFIRF 30 for adapting its filtering response. The difference y--y' is also put through a non-linear device 34 which has zero output for low level input signals. The non-linear device 34 is intended to remove residual errors between a true echo and its predicted value y'. If the signal y additionally contains a high level transmit signal, the high level signal passes through the non-linear device 34.
The echo canceller of FIG. 1, though superior to an echo suppressor, still suffers several drawbacks. The non-linear device 34 suppresses not only the residual echo but also the telephone line background noise when a high level signal is not present on the transmit line 20. It does however pass the noise in the presence of a high level signal. This effect can produce the impression that the telephone connection has been broken because of the sudden disappearance of all noise.
The echo canceller may be equipped with a double talk detector which disables the non-linear device 34, that is, removes the non-linearity, when the signal level on the receive side drops below a predetermined level. For instance, the bias point on a diode determines its non-linearity. Double-talk detectors are discussed by Horna in U.S. Pat. No. 4,360,712. If, however, the line is noisy, every pause in speech from the other station causes an increase of noise at the output of the disabled or linearized non-linear device 34. If the codec 26 is equipped with an adaptive quantization circuit for a constant ratio of signal to quantization noise or a voice recognition circuit for disconnecting unused transmit lines, the sudden increase in background noise can be detected as a speech signal and transmitted as speech despite its only being a noise burst.
Both of these problems have been known since echo suppressors have been used to control echoes. As a result, several devices were developed to measure the background noise and to then inject noncorrelated noise of the same RMS value at the output of the echo suppressor when the switch in the suppressor has broken the transmit line. The injection of noise prevents a change in the minimum signal level when operating the suppressor switch. An analog circuit of this type is described by Flanagan et al in U.S. Pat. No. 3,161,838. Crouse et al describe a more sophisticated circuit in U.S. Pat. No. 4,351,983. This circuit measures the background noise with digital processing. However, neither of these devices is directly applicable to circuits with echo cancellers. Both are relatively complex and have several limitations. They do not properly discriminate between background noise and echo signal and their turn-on time and operation speed are too slow.